Multifunctional autonomically healing composite material

ABSTRACT

A composite material, contains a polymer, a polymerizer, a corresponding catalyst for the polymerizer, and a plurality of capsules. The polymerizer is in the capsules. The composite material is self-healing.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] The subject matter of this application may in part have beenfunded by the Air Force (AFOSR Grant no. F49620-00-1-0094/White). Thegovernment may have certain rights in this invention.

BACKGROUND

[0002] The present invention relates to self-healing compositematerials.

[0003] Thermosetting polymers, used in a wide variety of applicationsranging from microelectronics to composite airplane wings, aresusceptible to damage in the form of cracking. Often these cracks formdeep within the structure where detection is difficult and repair isvirtually impossible. In fiber reinforced polymer composites, crackingin the form of fiber-matrix interfacial debonding, ply delamination, andsimple matrix cracking leads to degradation. In microelectronics,polymer encapsulates and polymer matrix composite printed circuit boardssuffer from similar forms of damage, but in addition to mechanicalfailure, cracks cause electrical failure of the component. Microcrackinginduced by thermal and mechanical fatigue is a longstanding problem inpolymer adhesives. Regardless of the application, once cracks haveformed within polymeric materials, the integrity of the structure issignificantly compromised. Typically, previously reported methods ofsuccessful crack healing require some form of manual intervention.

[0004] A proposed method of self-healing is described in “Self-HealingComposites Using Embedded Microspheres” D. Jung et al. Composites andFunctionally Graded Materials vol. MD-80, in Proceedings of the ASMEInternational Mechanical Engineering Conference and Exposition, 265-275(1997). The proposed method uses polyoxymethyleneurea (PMU) microspheresto store a crack filling agent to be released into the crack and rebondthe crack faces. The repair mechanism uses naturally occurringfunctional sites in a polyester matrix network to trigger the repairaction. Adding a reactive component to trigger the crack fillersolidification was specifically investigated in the case of embeddedepoxide components and embedded amine groups, and it was found that theamine groups did not retain sufficient activity and was determined to benot feasible. The PMU microcapsules used contained an epoxide monomer.

BRIEF SUMMARY

[0005] In a first aspect, the present invention is a composite material,containing: a polymer, a polymerizer, a corresponding catalyst for thepolymerizer, and a plurality of capsules. The polymerizer is in thecapsules.

[0006] In a second aspect, the present invention is a compositematerial, containing: a polymer, a polymerizer, a correspondingactivator for the polymerizer, and a first plurality of capsules. Thepolymerizer is in the capsules, and the corresponding activator is not anative activating moiety.

[0007] In a third aspect, the present invention is a method for makingthe above composites, including dispersing the capsules and thecorresponding catalyst or activator into the polymer.

[0008] Definitions

[0009] A polymerizer is a composition that will form a polymer when itcomes into contact with a corresponding activator for the polymerizer.Examples of polymerizers include monomers of polymers such as styrene,ethylene, (meth)acrylates, and dicyclopentadiene (DCPD); a monomer of amultimonomer polymer system such as diols, diamines, and epoxide; andprepolymers such as partially polymerized monomers still capable offurther polymerization.

[0010] An activator is anything that when contacted or mixed with apolymerizer will form a polymer. Examples of activators are catalysts,initiators, and native activating moieties. A corresponding activatorfor a polymerizer is an activator that when contacted or mixed with thatspecific polymerizer will form a polymer.

[0011] A catalyst is a compound or moiety that will cause apolymerizable composition to polymerize, and is not always consumed eachtime it causes polymerization. This is in contrast to initiators andnative activating moieties. Examples of catalysts include ring openingpolymerization (ROMP) catalysts such as Grubbs catalyst. A correspondingcatalyst for a polymerizer is a catalyst that when contacted or mixedwith that specific polymerizer will form a polymer.

[0012] An initiator is a compound that will cause a polymerizablecomposition to polymerize, and is always consumed at the time it causespolymerization. Examples of initiators are peroxides (which will form aradical to cause polymerization of an unsaturated monomer); a monomer ofa multi-monomer polymer system such as diols, diamines, and epoxide; andamines (which will form a polymer with an epoxide). A correspondinginitiator for a polymerizer is an initiator that when contacted or mixedwith that specific polymerizer will form a polymer.

[0013] A native activating moiety is a moiety of a polymer that whenmixed or contacted with a polymerizer will form a polymer, and is alwaysconsumed at the time it causes polymerization. Examples of a nativeactivating moiety is an amine moiety (which will form a polymer with anepoxide).

[0014] A compound is a molecule that contains at most 100 repeatingunits. This is in contrast to a polymer, which contains more than 100repeating units.

[0015] A capsule is a hollow closed object having an aspect ratio of 1:1to 1:10. The aspect ratio of an object is the ratio of the shortest axisto the longest axis; these axes need to be perpendicular. A capsule mayhave any shape that falls within this aspect ratio, such as a sphere, atoroid, or an irregular ameboid shape. The surface of a capsule may haveany texture, for example rough or smooth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Various other objects, features and attendant advantages of thepresent invention will be more fully appreciated as the same becomesbetter understood from the following detailed description whenconsidered in connection with the accompanying drawings in which likereference characters designate like or corresponding parts throughoutthe several views and wherein:

[0017]FIG. 1 illustrates an embodiment of a self-healing composite; and

[0018]FIG. 2 shows crack healing efficiency of the composite materials.

DETAILED DESCRIPTION

[0019] Further investigations by the group that published “Self-HealingComposites Using Embedded Microspheres” found that the use of naturalfunctionality was, in fact, not feasible. Approaches that had beenpreviously eliminated were reconsidered, resulting in the discovery thatsystems that do not use a corresponding native activating moiety willallow for a self-healing composite. Preferably, systems that use acatalyst added to the polymer are used. Not only can damage to thecomposite be repaired, but also in some cases the achieved healedstrengths are greater than the strength of the original matrix material.

[0020] The present invention includes a composite material, containingcapsules in a polymer. The capsules contain a polymerizer, and thecomposite material includes an activator that is not a correspondingnative activating moiety. Preferably, the activator is a correspondingcatalyst for the polymerizer. When a crack forms in the compositematerial, some of the capsules are broken, and the polymerizer movesinto the crack, coming into contact with the activator and forming apolymer. This repairs the crack.

[0021] The capsules contain a polymerizer. The polymerizer contains apolymerizable compound such as a monomer or prepolymer, and mayoptionally contain other ingredients, such as other monomers and/orprepolymers, stabilizers, solvents, viscosity modifiers such aspolymers, odorants, colorant and dyes, blowing agents, antioxidants, andco-catalysts. Preferably, the polymerizer is a liquid.

[0022] The polymer contains both capsules and a corresponding activatorfor the polymerizer. Preferably, the activator is a catalyst or aninitiator. Examples of polymerizable compounds are cyclic olefins,preferably containing 4-50 carbon atoms and optionally containingheteratoms, such as DCPD, substituted DCPDs, norbornene, substitutednorbornene, cyclooctadiene, and substituted cyclooctadiene.Corresponding catalysts for these are ring opening metathesispolymerization (ROMP) catalysts such as Schrock catalysts (Bazan, G.C.;Schrock, R.R.; Cho, H.-N.; Gibson, V.C. Macromolecules 24, 4495-4502(1991)) and Grubbs catalysts (Grubbs, R.H.; Chang, S. Tetrahedron 54,4413-4450 (1998)).

[0023] Another example of polymerizable compounds are lactones such ascaprolactone, and lactams, that when polymerized will form polyestersand nylons, respectively. Corresponding catalysts for these are cyclicester polymerization catalysts and cyclic amide polymerizationcatalysts, such as scandium triflate.

[0024] Furthermore, a polymerizer may contain a polymerizable compoundand one part of a two-part catalyst, with a corresponding initiatorbeing the other part of the two-part catalyst. For example, thepolymerizable compound may be a cyclic olefin; one part of a two-partcatalyst may be a tungsten compound, such as an organoammoniumtungstate, an organoarsonium tungstate, or an organophosphoniumtungstate; or a molybdenum compound, such as organoammonium molybdate,an organoarsonium molybdate, or an organophosphonium molybdate. Thesecond part of the two-part catalyst may be an alkyl aluminum compound,such as an alkoxyalkylaluminum halide, an aryloxyalkylaluminum halide,or a metaloxyalkylaluminum halide in which the metal in the compound istin, lead, or aluminum; or an organic tin compound, such as atetraalkyltin, a trialkyltin hydride, or a triaryltin hydride.

[0025] In another such system, the polymerizable compound may beunsaturated compounds such as acrylates; acrylic acids; alkyl acrylates;alkyl acrylic acids; styrenes; isoprene; and butadiene. In this case,atom transfer radical polymerization (ATRP) may be used, with one of thetwo components being mixed with the polymerizable compound and the otheracting as the initiator: one component being an organohalide such as1-chloro-1-phenylethane, and the other component could be a copper(I)source such as copper(I) bipyridyl complex. Alternatively, one componentcould be a peroxide such as benzoyl peroxide, and the other componentcould be a nitroxo precursor such as2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). These systems aredescribed in Malcolm P. Stevens; Polymer Chemistry: An Introduction, 3rdEdition; New York: Oxford University Press, 1999, p. 184-186

[0026] In another such system, the polymerizable compound may containisocyanate functional groups (—N═C═O) with hydroxyl functional groups(—OH). For this system, the polymerizable material may for example be acompound containing both an isocyanate group and a hydroxyl group, ortwo different compounds, one compound containing at least two isocyanategroups and the other compound containing at least two hydroxyl groups.The reaction between an isocyanate group and a hydroxyl group can form aurethane linkage (—N—C(═O)—O—) between the compounds, possibly releasingcarbon dioxide. This carbon dioxide can provide for the creation ofexpanded polyurethane foam; optionally the polymerizer may contain ablowing agent, for example a volatile liquid such as dichloromethane. Inthis case, condensation polymerization may be used, with one of the twocomponents being mixed with the polymerizable compound and the otheracting as the initiator: for example, one component could be an alkyltincompound such as stannous 2-ethylhexanoate, and the other componentcould be a tertiary amine such as diazabicyclo[2.2.2]octane (DABCO).These systems are described in Malcolm P. Stevens; Polymer Chemistry: AnIntroduction, 3rd Edition; New York: Oxford University Press, 1999, p.378-381.

[0027] Optionally, the activator, such as the catalyst or initiator mayalso be in a separate set of capsules. Furthermore, this separate set ofcapsules may also contain stabilizers, solvents, viscosity modifierssuch as polymers, odorants, colorant and dyes, blowing agents,antioxidants, and co-catalysts. Optionally, a set of capsules may bepresent that contain one or more additional ingredients, such asstabilizers, solvents, viscosity modifiers such as polymers, odorants,colorant and dyes, blowing agents, antioxidants, and co-catalysts.

[0028] The capsules contain a polymerizer. Preferably, the capsules havean average diameter of 10 nm to 1 mm, more preferably 30-500 μm, mostpreferably to 50-300 μm. The capsules have an aspect ratio of 1:1 to1:10, preferably 1:1 to 1:5, more preferably 1:1 to 1:3, and even morepreferably 1:1 to 1:2, and most preferably 1:1 to 1:1.5.

[0029] The wall thickness of the capsules is preferably 100 μm to 3 μm.The selection of capsule walls thickness depends on the polymer in thecomposite. For example, capsule walls that are too thick will notrupture when a crack approaches, while capsules with very thin wallswill break during processing.

[0030] The adhesion between the capsules and the polymer of thecomposite influences whether the capsules will rupture or debond in thepresence of an approaching crack. To promote the adhesion between thepolymer and capsule wall, various silane coupling agents may be used.Typically, these are compounds of the formula R-SiX₃ Where R ispreferably a reactive group R¹ separated by a propylene group fromsilicon, and X is an alkoxy group (preferably methoxy), such as R¹CH₂CH₂CH₂Si(OCH₃)₃. Examples include silane coupling agents availablefrom DOW CORNING (with reactive group following the name inparentheses): Z6020 (Diamino); Z6030 (Methacrylate); Z6032 (StyrylamineCationic); Z6040 (Epoxy); and Z6075 (Vinyl).

[0031] To increase the adhesion between the capsules and a polymer inthe composite, the capsules may be treated by washing them in a solutionof the coupling agent. For example, urea-formaldehyde capsules may bewashed in a solution of Silane Z6020 or Z6040 and hexane (1:20 wt.)followed by adding Silane Z6032 to the polymer (1% wt.).

[0032] Capsules may be made by a variety of techniques, and from avariety of materials, such as those described in Microencapsulation:Methods and Industrial Applications Ed. Benita, Simon Marcel Dekker, NewYork, 1996; Microencapsulation: Processes and Applications Ed.Vandegaer, J. Plenum Press, New York, 1974; and Microcapsule Processingand Technology Kondo, A. Marcel Dekker, New York, 1979. Examples ofmaterials from which the capsules may be made, and the techniques formaking them include: urea-formaldehyde, formed by in situpolymerization; gelatin, formed by complex coacervation; polyurea,formed by the reaction of isocyanates with a diamine or a triamine,depending on the degree of crosslinking desired (the extent ofcrosslinking also determines the brittleness of the capsule);andpolyamide, formed by the use of a suitable acid chloride and a watersoluble triamine.

[0033] The polymer may be any polymeric material into which the capsulesmay be dispersed. Examples include polyamides such as nylons; polyesterssuch as poly(ethylene terephthalate) and polycaprolactone;polycarbonates; polyethers such as epoxides; polyimides such aspolypyromellitimide (for example KAPTAN); phenol-formaldehyde resins(for example BAKELITE); amine-formaldehyde resins such as a melamineresin; polysulfones; poly(acrylonitrile-butadiene-styrene) (ABS);polyurethanes; polyolefins such as polyethylene, polystyrene,polyacrylonitrile, polyvinyls, polyvinyl chloride, poly(DCPD) andpoly(methyl methacrylate); polysilanes such as poly(carborane-siloxane);and polyphosphazenes.

[0034] The capsules and activator (such as the catalyst or initiator)may be dispersed into the polymer by forming the polymer around thecapsules and activator, such as by polymerizing monomer to form thepolymer with the capsules and activator mixed into the monomer.Particularly in the case of catalysts, the catalyst may serve as both acatalyst for the polymer and as the corresponding activator for thepolymerizer in the capsules. Examples of this system include DCPD as thepolymerizer, the polymer is poly(DPCD), and a Grubbs catalyst serves toform the poly(DPCD) and acts as the activator for the DCPD in thecapsules; and caprolactone as the polymerizer, the polymer ispoly(caprolactone), and scandium triflate acts as the activator for thecaprolactone in the capsules.

[0035] Alternatively, the polymer may be first formed, and then thecapsules and activator mixed in. For example, the polymer may bedissolved in a solvent and the capsules and activator mixed into thesolution, followed by removal of the solvent. The activator may becoated onto the capsules prior to dispersing the capsules into thepolymer. Furthermore, other components may be added to the polymer, suchas fibers, fillers, adhesion modifiers, blowing agents, anti-oxidants,colorants and dyes, and fragrances.

[0036]FIG. 1 illustrates an embodiment of a self-healing composite. Anapproaching crack ruptures embedded capsules (referred to asmicrocapsules in the figure) releasing polymerizer (referred to ashealing agent in the figure) into the crack plane through capillaryaction. Polymerization of the healing agent may be triggered by contactwith the activator (here a catalyst), bonding the crack faces. Thedamage-induced triggering mechanism provides site-specific autonomiccontrol of the repair. As shown in FIG. 1, an encapsulated healing agentis embedded in a structural composite matrix containing a catalystcapable of polymerizing the healing agent: (i) cracks form in the matrixwherever damage occurs, (ii) The crack ruptures the microcapsules,releasing the healing agent into the crack plane through capillaryaction, (iii) The healing agent contacts the catalyst triggeringpolymerization that bonds the crack faces closed.

EXAMPLES

[0037] The following examples and preparations are provided merely tofurther illustrate the invention. The scope of the invention is notconstrued as merely consisting of the following examples.

[0038] General Procedure for Preparation of Capsules by In SituPolymerization

[0039] In a 600 mL beaker is dissolved urea (0.11 mol, 7.0 g) followedby resorcinol (0.5 g) and ammonium chloride (0.5 g) in water (150 ml). A5 wt.% solution of ethylene maleic anhydride copolymer (100 mL) is addedto the reaction mixture, and the pH of the reaction mixture is adjustedto 3.5 using 10% NaOH solution. The reaction mixture is agitated at 454rpm, and to the stirred solution is added 60 mL of dicyclopentadiene toachieve an average droplet size of 200 μm. To the agitated emulsion isadded 37% formaldehyde (0.23 mol, 18.91 g) solution, and then thetemperature of the reaction mixture is raised to 50° C. and maintainedfor 2 h. After 2 h, 200 mL of water is added to the reaction mixture.After 4 h, the reaction mixture is cooled to room temperature, andcapsules are separated. The capsule slurry is diluted with an additional200 mL of water and washed with water (3×500mL). The capsules areisolated by vacuum filtration, and air-dried. Yield:80%. Averagesize:220 μm.

[0040] Composite Epoxy Specimen Manufacture

[0041] The epoxy matrix composite was prepared by mixing 100 parts EPON828 (Shell Chemicals Inc.) epoxide with 12 parts DETA(diethylenetriamine) curing agent (Shell Chemicals Inc.). Compositeepoxy specimens were prepared by mixing 2.5% (by wt.) Grubbs' catalystand 10% (by wt.) capsules with the resin mixture described above. Theresin was then poured into silicone rubber molds and cured for 24 h atroom temperature, followed by postcuring at 40 ° C. for 24 h.

[0042] Example

[0043] DCPD filled capsules (50-200 μm average diameter) with aurea-formaldehyde shell were prepared using standard microencapsulationtechniques. The capsule shell provides a protective barrier between thecatalyst and DCPD to prevent polymerization during the preparation ofthe composite.

[0044] The reaction scheme for the polymerization of DCPD is shown below

[0045] To assess the crack healing efficiency of these compositematerials, fracture tests were performed using a tapereddouble-cantilever beam (TDCB) specimen (FIG. 2). Self-healing compositeand control samples were fabricated. Control samples consisted of: (1)neat epoxy containing no Grubbs' catalyst or capsules, (2) epoxy withGrubbs' catalyst but no capsules and (3) epoxy with capsules but nocatalyst. A sharp pre-crack was created in the tapered samples by gentlytapping a razor blade into a molded starter notch. Load was applied in adirection perpendicular to the pre-crack (Mode I) with pin loading gripsas shown in FIG. 2. The virgin fracture toughness was determined fromthe critical load to propagate the crack and fail the specimen. Afterfailure, the load was removed and the crack allowed to heal at roomtemperature with no manual intervention. Fracture tests were repeatedafter 48 hours to quantify the amount of healing.

[0046] A representative load-displacement curve is plotted in FIG. 2demonstrating ca. 75% recovery of the virgin fracture load. In greatcontrast, all three types of control samples showed no healing and wereunable to carry any load upon reloading. A set of four independentlyprepared self-healing composite samples showed an average healingefficiency of 60%. When the healing efficiency is calculated relative tothe critical load for the virgin, neat resin control (upper horizontalline in FIG. 2), a value slightly greater than 100% is achieved. Theaverage critical load for virgin self-healing samples containingcapsules and Grubbs' catalyst was 20% larger than the average value forthe neat epoxy control samples, indicating that the addition of capsulesand catalyst increases the inherent toughness of the epoxy.

[0047] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A composite material, comprising: (i) a polymer, (ii) a polymerizer,(iii) a corresponding catalyst for the polymerizer, and (iv) a pluralityof capsules, wherein the polymerizer is in the capsules.
 2. Thecomposite material of claim 1, wherein the polymerizer comprises atleast one monomer selected from the group consisting of cyclic olefins,lactones, lactams, acrylates, acrylic acids, alkyl acrylates, alkylacrylic acids, styrenes, isoprene and butadiene.
 3. The compositematerial of claim 1, wherein the polymerizer comprises cyclic olefins.4. The composite material of claim 1, wherein the polymer comprises atleast one member selected from the group consisting of polyamides,polyesters, polycarbonates, polyethers, polyimides, phenol-formaldehyderesins, amine-formaldehyde resins, polysulfones,poly(acrylonitrile-butadiene-styrene), polyurethanes, polyolefins, andpolysilanes.
 5. The composite material of claim 1, wherein the polymercomprises at least one member selected from the group consisting ofpolyesters and polyethers.
 6. The composite material of claim 1, whereinthe corresponding catalyst for the polymerizer comprises at least onemonomer selected from the group consisting of ROMP catalysts and cyclicester polymerization catalysts.
 7. The composite material of claim 1,wherein the corresponding catalyst for the polymerizer comprises a ROMPcatalyst.
 8. The composite material of claim 1, wherein the capsuleshave an aspect ratio of 1:1 to 1:2, and an average diameter of 10 nm to1 mm.
 9. The composite material of claim 1, wherein the capsulescomprise a polymer of urea and formaldehyde, gelatin, polyurea, andpolyamide.
 10. The composite material of claim 1, wherein thepolymerizer comprises DCPD, the polymer comprises epoxy, thecorresponding catalyst for the polymerizer comprises a Grubbs catalyst,the capsules have an aspect ratio of 1:1 to 1:1.5, and an averagediameter of 30-300 μm, and the capsules comprise a polymer of urea andformaldehyde.
 11. The composite material of claim 1, wherein thepolymerizer comprises DCPD, the polymer comprises poly(DCPD), thecorresponding catalyst for the polymerizer comprises a Grubbs catalyst,and the capsules have an aspect ratio of 1:1 to 1:1.5, and an averagediameter of 30-300 μm.
 12. The composite material of claim 1, whereinthe polymerizer comprises caprolactone, the polymer comprisespoly(caprolactone), the corresponding catalyst for the polymerizercomprises a scandium triflate, and the capsules have an aspect ratio of1:1 to 1:1.5, and an average diameter of 30-300 μm.
 13. A compositematerial, comprising: (i) a polymer, (ii) a polymerizer, (iii) acorresponding activator for the polymerizer, and (iv) a first pluralityof capsules, wherein the polymerizer is in the capsules, and thecorresponding activator is not a native activating moiety.
 14. Thecomposite material of claim 13, wherein said corresponding activator isa corresponding initiator for the first polymerizer.
 15. The compositematerial of claim 13, further comprising a second plurality of capsules,wherein said activator is in the second plurality of capsules.
 16. Thecomposite material of claim 14, wherein said corresponding activator isa monomer.
 17. The composite material of claim 13, wherein saidpolymerizer comprises a monomer and a first part of a two-part catalyst,and said corresponding activator is a second part of the two-partcatalyst.
 18. The composite material of claim 13, wherein the polymercomprises at least one member selected from the group consisting ofpolyamides, polyesters, polycarbonates, polyethers, polyimides,phenol-formaldehyde resins, amine-formaldehyde resins, polysulfones,poly(acrylonitrile-butadiene-styrene), polyurethanes, polyolefins, andpolysilanes.
 19. The composite material of claim 13, wherein the polymercomprises at least one member selected from the group consisting ofpolyesters and polyethers.
 20. The composite material of claim 13,wherein the capsules have an aspect ratio of 1:1 to 1:2, and an averagediameter of 10 nm to 1 mm.
 21. The composite material of claim 13,wherein the capsules comprise a polymer of urea and formaldehyde,gelatin, polyurea, and polyamide.
 22. A method for making the compositeof claim 1, comprising: dispersing the capsules and the correspondingcatalyst into the polymer.
 23. A method for making the composite ofclaim 13, comprising: dispersing the capsules and the correspondingactivator into the polymer.